Notes # _____
Chapter 25 Notes
Date: _______
25.1: Nuclear Radiation
Recall from Chapter 4 that the atomic nucleus is comprised of two particles: the proton
and the neutron. In some atoms, the ratio of protons to neutrons causes the nucleus to
become unstable, causing ______________________. In the late 1800’s and early 1900’s a few
scientists, namely Henri Becquerel and his young colleagues, Marie and Pierre Curie,
discovered that the element uranium caused problems with photographic plates. Further
research led to the discovery of _________________. In chemical reactions, ________________
are responsible for the interaction between atoms; in nuclear reactions it is the __________.
Unstable nuclides, called ______________________ become more stable as they ______________
energy stored in the nucleus. Two other differences between chemical and nuclear
reactions: unlike chemical reactions, nuclear reactions are unaffected by changes in
_______________________ or _______________________, or the use of _________________________
(chemicals that affect the rate of a reaction but are not consumed during the reaction); and
radioactive decay requires no energy to initiate the decay, it spontaneously occurs.
3 Types of Radiation
Each of the three types of decay produces a new atom and releases from the nucleus a
different particle. The least dangerous ( or _____________________) is ____________ radiation.
An alpha particle (recall Rutherford’s experiment) consists only of two protons and two
neutrons and has a _____ charge. A common method of writing an alpha particle is either
______ or the preferred _______. Notice that both leave out the charge. A well known
example of alpha radiation occurs with uranium-238:
Note that in alpha decay, the atomic number decreases by _____ and the mass number
decreases by _____.
The second type of radiation occurs when a neutron breaks down into a _____________ and a
________ particle in the reaction:
The ________ particle is really a fast moving ______________ that is ejected from the nucleus
and escapes from the atom. In beta decay the atomic number increases by ______ but the
mass number remains the same. The best known example of beta decay occurs with carbon14 (useful in carbon dating):
Because a beta particle is lighter and has half the charge of an alpha particle it penetrates
further and therefore is more dangerous. A single piece of paper could stop an alpha
particle, but it takes at least a piece of aluminum foil or wood to stop a beta particle.
The highest energy and therefore most dangerous form of radiation is called _____________
radiation. A __________ _______ (
) is an extremely high energy _____________ (a form of
__________________ __________________). This radiation is usually emitted during alpha or
beta decay and can cause tremendous damage to human tissue and is very difficult to
shield against since gamma rays have no mass nor charge. A wall of lead or concrete many
feet thick is required to shield against gamma radiation. Nuclear power plants must protect
against this dangerous energy release. As an example, thorium-239 undergoes alpha decay
while releasing some energy in the form of gamma radiation:
Note that by itself gamma decay does not change the composition of the nucleus, it just
releases some energy. Gamma rays are very common in outer space, making it a very
dangerous place to travel. Without the protection of our magnetic field, life on Earth would
be nearly impossible.
Practice. Determine the products of the following:
Alpha Decay
radon-222:
thorium-232:
Beta Decay
copper-66:
tin-94:
Notes # _____
Chapter 25 Notes, part II
Date: _______
25.2: Nuclear Transformations
There are more than 2500 known nuclides in the universe; only about _____% are stable.
When scientists plot the ratio of neutrons to protons of these stable isotopes the following
graph is seen (the dots represent the stable isotopes):
The straight line represents:
For low atomic numbers (below about 20 protons):
As Z increases above 20 protons:
Nuclei with _____ numbers of both protons and neutrons are generally more stable than
those with ____ numbers of these particles. Nuclei that contain certain specific numbers of
protons and neutrons within a nucleus have an extra degree of stability. These so-called
_________ ___________ for protons and for neutrons are 2, 8, 20, 28, 50, 82, and 126. Also,
nuclei with even numbers of protons or neutrons are generally more stable than those with
odd numbers of these particles. If a nucleus is unstable an atom may undergo radioactive
decay.
The ratio of neutrons to protons will determine the type of decay:
(1) If there are too many neutrons relative to protons the atom will undergo _______
emission. Recall that during this process a neutron is turned into a proton and an
electron:
example:
(2) If there are too many protons relative to neutrons the atom will undergo ____________
______________. During this process an electron is captured by an nucleus and a proton
is then converted into a neutron:
example:
(3) Another process can occur if there are too many protons relative to neutrons, called
_____________ ______________. Here a proton converts into a neutron and releases a
antimatter particle that is exactly like an electron, but with a positive charge:
example:
(4) Lastly, for elements # ______ and above, all of which have too many protons and
neutrons, the process is usually ___________ _______________, which also increases the
0
+
n to p ratio:
example:
Further information and an online practice quiz can be found at:
http://algebralab.com/practice/practice.aspx?file=Reading_TheBandOfStability.xml
Half-Life
Radioisotopes break down in differing amounts of time, some very short (a few billionths of
seconds), others incredibly long (hundreds of millions of years). The amount of time it takes
any one atom to mutate is difficult to measure so instead an average life span is used,
called its ______________. By definition, the half-life ( t1/2 ) of a radionuclide is equal to the
amount of time it takes for half of the original sample to decay.
Examples:
(1) Carbon-14 has a half-life of 5730 years. How much of an original 100.0 g sample
would remain after 22,920 years?
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(2) U-238 has a half-life of 4.46 x 10 years. How much U-238 should be present in a
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sample 2.5 x 10 years old, if 2.00 grams was present initially?
(3) A sample of strontium-91 has decayed from 20.0 grams to 2.50 grams in 1800 days.
Calculate the half-life of strontium-91.
Transmutation
Any time that one element changes to another element it is called ____________________.
Naturally, radioactivity produces a new element. In the lab, or in a nuclear reactor, another
process can produce a new element. When one nucleus is ____________________ by another
particle (usually a proton, neutron or alpha particle, but sometimes a heavier nucleus) it
might absorb this new nucleus, maybe only for a short period of time. All elements beyond
element #_______ (called the _______________________ elements) are produced this way. Many
of these elements were created at _______________ by Glenn Seaborg, et al, and were named
to honor many iconic scientific symbols (look at elements #95-104 &106 and see if you can
figure out who or what they are named for).
Example 1:
When Rutherford first bombarded nitrogen-14 with an alpha particle an unstable
fluorine-18 atom was produced that quickly decayed into a more stable oxygen-17 plus
one other particle. Set up the complete reaction and determine the missing particle:
Example 2:
When a uranium-235 nucleus is struck by a neutron it rapidly decomposes into a
barium-144 nucleus, a krypton-89, and a number of neutrons. Write the reaction and
determine the number of neutrons produced.
In the above reaction, since more neutrons are produced than are put in, a ___________
________________ is started. When one uranium-235 decays energy is released, and even
more neutrons are produced that can then bombard other U-235 nuclei, releasing even
more energy, and so on:
Here is the complete decay series for uranium-238. There are 14 steps that eventually lead
to a stable nuclide, lead-206:
For further examples see:
http://www.mhhe.com/physsci/chemistry/chang7/ssg/chap23_1sg.html
Notes # _____
Chapter 25 Notes, Part III
Date: _______
25.3: Fission and Fusion
When heavy atomic nuclei are bombarded they break apart in a process called ____________.
There are only two fissionable isotopes: ___________________ and _____________________.
During fission some of the mass of these materials is “lost” as energy shown in the famous
equation: _____________. The nuclear energy stored in 1 kilogram of radioactive uranium is
equivalent to the chemical energy stored in approximately _____________________ of coal.
This represents a vast source of energy!
Since fission is a chain reaction it is critical that in a nuclear power plant the neutrons are
controlled via two processes: _______________________ and ________________________.
During moderation neutrons are slowed when they enter water or graphite. If the neutrons
are not slowed they will travel too fast to be absorbed by another nucleus and the chain
reaction will stop. However if the neutron chain reaction gets too rapid the temperature
can climb to critical temperatures and possibly even explode. Neutron absorption uses
_______________ _________, made from the element _____________________ that absorb excess
neutrons. These control rods are raised and lowered into the ______________ _________ to
regulate the amount of neutron interaction.
Nuclear Waste
Nuclear power plants are not without serious environmental issues. One major problem is
with _________ ________ _______. As the reaction occurs in a power plant these fuel rods
(either U-235 or Pu-239) become less potent and are no longer useable, but they still
contain a tremendously dangerous level of radiation. These spent fuel rods are usually
stored on site in large holding tanks of water for long periods of time to allow for further
radioactive decay. The transportation and disposal of these rods as well as the security risk
involved have led to a tremendous amount of worry, debate and expense in countries that
use nuclear power. In the United States, Hanford, WA and Yucca Mountain, NV as well as
smaller locations throughout the country are the locations of vast amounts of nuclear
waste, and will be for millennia.
Fusion
In many ways fusion is the opposite of fission. For very light elements, especially
_____________, very high pressures can lead to the combining of nuclei into slightly heavier
elements, releasing the excess mass as energy. In the case of hydrogen the process is:
The problem with this reaction is that the pressure and temperature needed to produce
fusion (greater than _________________°C!) are so extreme that it has so far been an
uncontrollable reaction. Only in _____________________ devices have humans been able to
“use” this energy. Around the planet may large research labs have been attempting to
achieve controllable fusion for the past few decades. If controlled fusion is ever really
achieved it would be an incredible milestone for science and humanity due to the
tremendous amount of cheap, clean energy it could provide.
In stars the process gets a little more complex. At the center of a young protostar, called its
________, the pressures and temperatures become great enough as the protostar collapses
that eventually fusion begins to occur and a “________ _____________” star forms. For
hundreds of millions to many billions of years and star can run on its hydrogen fuel alone.
In the case of our sun, it has been fusing hydrogen for 4.5 billion years and has enough left
for another _________________ years. But stars always run out of hydrogen and the core
shrinks and heats up. Once the core temperature has risen to 100,000,000 K, the helium in
the core starts to fuse, through the ___________________ process:
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The Be nucleus is highly unstable, and will decay in about 10–12 s unless an alpha
particle fuses with it first. This is why high temperatures and densities are necessary. As a
star continues in its life cycle it consumes more and more of its atomic fuel reserve and the
core shrinks and gets hotter allowing even heavier elements to fuse. Eventually when a
temperature of _________________ K is reached the ultimate atomic nucleus is produced:
__________. The star can no longer fuse, the core collapses and a tremendous shockwave,
called a _________________, rips the star apart. It is during this process that every element
up to uranium was produced in the universe. In fact, every element heavier than helium
was produced in a star. So, if you think about it: “you are stardust...” (Carl Sagan said it
first.)